Dendritic cells (DCs) comprise a multivariate family of cell types whose principal function is in the primary initiation of immune responses. Blood or myeloid-derived dendritic cells (MDDCs) patrol areas of the body that are susceptible to invasion by pathogens and engulf antigens for later processing and presentation to T lymphocytes. HIV has apparently appropriated this feature of the immune system to better establish and maintain infection of its primary target--CD4 positive T cells. MDDCs can efficiently bind and transfer HIV infectivity without themselves becoming infected. Under conditions of limiting soluble virus, MDDCs greatly enhance infection of target cells. DCs are therefore potentially important both in primary infection and in the persistence of drug-resistant reservoirs in infected individuals, making them an attractive target for new therapies aimed at prevention and treatment of HIV infection. We have begun to examine the trafficking of HIV between DCs and T cells using our unique system of directly visualizing HIV in cells by tagging virions with the green fluorescent protein. Live cell video microscopy revealed that HIV is recruited to sites of cell contact in MDDCs, even when surface HIV is removed by proteolysis. Analysis of conjugates between MDDCs and autologous T cells showed that, even in the absence of antigen-specific signaling, the HIV receptors CD4 and CXCR4 on the T cell are recruited to the interface and a significant proportion of MDDCs concentrate HIV to the same region. Because this structure bears similarities to the antigen signaling structure known as the immunological synapse, we refer to it as the infectious synapse. The experiments proposed here will explore the dynamics of the formation the infectious synapse and examine the signaling events responsible for its formation. Additionally, the efficacy of a new class of HIV anti-viral compounds that target HIV fusion and entry will be assessed, providing important information about potential resistance to this therapeutic approach. Finally, the mechanism of transmission will be investigated, focusing on whether HIV is displayed on the DC surface prior to infection of the target, or if it is secreted from the cell in the form of exosomes. Elucidation of this mechanism will be important for the design of new anti-viral compounds that might ultimately lead to the eradication of this drug-resistant pool of infectious HIV.